logtail: count the message a hung sender check defers in cleanup (audit M13)

Seen on the e2e stand, the panel frozen with SIGSTOP once DATA was
answered: the client gets 451 4.7.1 after 30 s, and the warning that names
the socket is cleanup's, not smtpd's:

  postfix/cleanup[1339]: warning: milter unix:/run/selfpost/sender.sock:
    can't read SMFIC_HEADER reply packet header: Connection timed out
  postfix/cleanup[1339]: 7BCAF3C75DA0: milter-reject: END-OF-MESSAGE ...
    4.7.1 Service unavailable - try again later; ...

The counter read smtpd lines only, so Health's "messages deferred" missed
the one case that tells a hung panel from a stopped one. milterLineRe now
takes smtpd and cleanup; cleanup's "milter unix:<socket>:" warning counts a
message each time (one cleanup process serves message after message, and the
first warning ends the message); smtpd keeps once per connection. The
composed fixture is replaced by the seen lines, with two messages through one
cleanup process and OpenDKIM hung in cleanup (not counted).

e2e panel_hung_mid_message_is_counted: freezes the panel after DATA's 354,
wants a 4xx at the end of DATA, logs what mail.log says, and waits for
Health's today count to grow. kill through sh -c: the image has no kill(1).

CHANGELOG [Unreleased], audit plan line ticked, roadmap 13/61.
Evidence on the test server for this tree (with the next commit's docs):
go vet clean, go test ./... 23 packages ok, make e2e exit 0 -
renewed_certificate_picked_up 1.2 s, panel_hung_mid_message_is_counted
31.7 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/main_test.go

@@ -310,6 +310,9 @@ func TestE2E(t *testing.T) {
 	run("panel_down_defers_mail", func(t *testing.T) {
 		testPanelDownDefersMail(t, sc)
 	})
+	run("panel_hung_mid_message_is_counted", func(t *testing.T) {
+		testPanelHungMidMessage(t, sc)
+	})
 	run("session_survives_restart", func(t *testing.T) {
 		testSessionSurvivesRestart(t, sc)
 	})